Ok, dumb question follows (get out now if you cant stand another one!)

Was thinking about getting an M16 type, to relive some not so fond army memories. Yeah, Im a glutton for punishment.
anyway, went to gunbroker and saw all shapes,sizes and prices.
what caught my eye was all the listings for upper 1/2,lower1/2,trigger assemblies,stocks,grips etc. One guy even has a lower assembly w/o trigger mechanism holes (for a full auto build ?)
seems like i can order all components for a "custom" weapon. and assemble myself.
so, what am i missing here? can i really build my own?
thanks
tom